Traditional computers which receive a sequence of instructions and execute the sequence one instruction at a time are known. The instructions executed by these computers operate on single-valued objects, hence the name "scalar" for these computers.
The operational speed of traditional scalar computers has been pushed to its limits by advances in circuit technology, computer mechanisms, and computer architecture. However, with each new generation of competing machines, new acceleration mechanisms must be discovered for traditional scalar machines.
A recent mechanism for accelerating the computational speed of uni-processors is found in reduced instruction set architecture that employs a limited set of very simple instructions. Another acceleration mechanism is complex instruction set architecture which is based upon a minimal set of complex multi-operand instructions. Application of either of these approaches to an existing scalar computer would require a fundamental alteration of the instruction set and architecture of the machine. Such a far-reaching transformation is fraught with expense, downtime, and an initial reduction in the machine's reliability and availability.
In an effort to apply to scalar machines some of the benefits realized with instruction set reduction, so-called "superscalar" computers have been developed. These machines are essentially scalar machines whose performance is increased by adapting them to execute more than one instruction at a time from an instruction stream including a sequence of single scalar instructions. These machines typically decide at instruction execution time whether two or more instructions in a sequence of scalar instructions may be executed in parallel. The decision is based upon the operation codes (OP codes) of the instructions and on data dependencies which may exist between instructions. An OP code signifies the computational hardware required for an instruction. In general, it is not possible to concurrently execute two or more instructions which utilize the same hardware (a hardware dependency) or the same operand (a data dependency). These hardware and data dependencies prevent the parallel execution of some instruction combinations. In these cases, the affected instructions are executed serially. This, of course, reduces the performance of a super scalar machine.
Superscalar computers suffer from disadvantages which it is desirable to minimize. A concrete amount of time is consumed in deciding at instruction execution time which instructions can be executed in parallel. This time cannot be readily masked by overlapping with other machine operations. This disadvantage becomes more pronounced as the complexity of the instruction set architecture increases. Also, the parallel execution decision must be repeated each time the same instructions are to be executed.
In extending the useful lifetime of existing scalar computers, every means of accelerating execution is vital. However, acceleration by means of reduced instruction set architecture, complex instruction set architecture, or superscalar techniques is potentially too costly or too disadvantageous to consider for an existing scalar machine. It would be preferred to accelerate the speed of execution of such a computer by parallel, or concurrent, execution of instructions in an existing instruction set without requiring change of the instruction set, change of machine architecture, or extension of the time required for instruction execution.